The present invention relates to an apparatus and method for bonding optical element to a support and, more particularly, to such bonding which is unaffected by any differential coefficients of thermal expansion of the optical element, the support and the material bonding the element to the support.
Optical glass and like optical elements are conventionally bonded to a support by bonding materials. Typical bonding materials have a coefficient of thermal expansion which is approximately ten times higher than that of the optical element or the substrate to which the element is bonded. As a consequence, temperature changes cause differential expansion of the bond and thereby results in optical shifts in the optical element. In addition, if the bond interface is not exactly uniform in thickness, the thicker part expands more than the thinner part, to create a wedge which results further in an optical shift.
Sometimes, even if the bond thicknesses are made precisely uniform throughout the bonding area, the bonding material buckles, which causes the optical element to tilt with respect to its support, and produce an optical shift.
Such thermally induced optical shifts are highly undesirable as causing optical misalignment in precise optical equipment.
One common method of minimizing such optical shifts and optical misalignment requires the selection of bonding materials which tend to minimize such shifts. In addition, the bond thickness is made as small as possible so that its effect is minimized. Finally, some techniques employ a boresight maneuver immediately prior to use of the equipment first to measure the optical shift and then to correct it either electronically or physically by tilting mirrors, in order that the optical shift be nulled out. Such approaches significantly affect the overall cost and complexity of the system.
An additional problem occurs when a system includes two or more optical elements, for example, prisms, which are to be aligned one with respect to the other. Positioning of such multiple image prisms requires that each prism be positioned precisely with respect to other prisms. Conventionally, this requires some form of support and the attendant bonding of the prisms thereto. Such bonding techniques, such as referred to above, create difficulties in precise alignment. Typically, the prisms are aligned one with respect to the other in a holding fixture that includes supporting the prism frame. Once the prisms are correctly positioned with respect to the frame, the bonding material is used to bond the prisms to the support structure. Because the prism surfaces are not necessarily parallel to the walls of the frame, the bond thickness will vary over the bond area, with the result that the prism alignment shifts with temperature as a result of the wedge and temperature buckling effects.